How to create a bottom for this rock to make it a whole mesh?
You could copy some of your rock mesh (selection with inner region or circle select...) and orient it, so that you don't need to fill a large hole, but you have a gap between your rock mesh and that patch. 

Then you could fill this gap with methods like shortest path select (hold ctrl while selecting) the gaps borders for short lengths of the gap and then fill faces (with F). 

The gap filling result will be nicest if in advance to filling you extrude the parts of the gap closer to each other in multiple iterations(Then you have quads in most of the gap region)